Bombesin-related peptides ([gastrin-releasing peptide [GRP], neuromedin B) interact with two distinct receptors (GRP-R, NMB- R) to mediate a number of effects in the gastrointestinal tract (GI), central nervous sytem (CNS) and on growth of normal and neoplastic tissues. Furthermore, two related receptors, a mammalian orphan receptor (BRS-3), having 60% homology to GRP-R and a novel receptor in amphibians, BB-4-R has been described recently. The aims of this project are to understand the pharmacology, molecular pharmacology, and cell biology of these receptors as well as to develop specific agonists and antagonists that can be used to determine their physiological roles. Investigations being performed include expression of these receptors in stable cell lines that resemble native receptors in their cell biology and pharmacology; investigations using site-directed mutagenesis and receptor chimeras to define receptor structural determinants of ligand selectivity and specificity for agonists and antagonists, pharmacological studies of BN-related peptides to identify selective agonists/antagonists and studies of native cells and transfected cells to define the transduction cascades of these receptors. Using receptor modeling and mutagenesis studies we have recently designed a selective agonist for the BRS-3 receptor (J.Biol.Chem. 276, 9219-9229, 2001), as well as characterized the molecular basis for the selectivity of two high affinity peptide antagonists for the GRP receptor (J. Biol. Chem. 276, 36652-36663, 2001). Prior to the BRS-3 study, no selective ligands existed for the BRS-3 receptor restricting the ability to investigate its physiological roles. Using conformationally restricted analogues of [B-Ala112] bombesin and modeling we identified an amino-3 phenyl propionic acid analogue as the first selective BRS-3 agonist. In the second study the presence of three amino acids in the 4th extracellular domain (EC) of the GRP receptor were identified as the critical residues for the antagonists selectivity and this selectivity was mediated by cation-pi interactions and hydrogen binding.